A quarter-wave plate that converts linearly polarized light into circularly polarized light, a half-wave plate that changes the plane of vibration of linearly polarized light by 90°, and the like are known. These retardation films can achieve accurate conversion of specific monochromatic light so that ¼λ or ½λ retardation occurs.
However, known retardation films have a problem in that polarized light that passes through is converted into colored polarized light. Specifically, since a material that forms the retardation film has wavelength dispersion with respect to retardation, and a polarization state distribution corresponding to each wavelength occurs with respect to white light that includes different light rays in the visible region, it is impossible to achieve accurate ¼λ or ½λ retardation over the entire wavelength band.
In order to solve the above problem, various wideband retardation films that can achieve uniform retardation with respect to light over a wide wavelength band (i.e., retardation films having reverse wavelength dispersion) have been studied (see Patent Documents 1 to 6, for example).
It has been desired to reduce the thickness of a flat panel display as much as possible along with an improvement in performance and widespread use of mobile information terminals (e.g., mobile personal computers and mobile phones). Therefore, a reduction in thickness of the retardation film has also been desired.
It has been considered that it is most effective to produce a retardation film by applying a polymerizable composition that includes a low-molecular-weight polymerizable compound to a film substrate in order to reduce the thickness of the retardation film. Various low-molecular-weight polymerizable compounds having excellent wavelength dispersion, and various polymerizable compositions using such polymerizable compounds have been developed (see Patent Documents 7 to 24, for example).
However, the low-molecular-weight polymerizable compounds or the polymerizable compositions disclosed in Patent Documents 7 to 24 have a number of problems in that reverse wavelength dispersion may be insufficient, or it may be difficult to apply the low-molecular-weight polymerizable compounds or the polymerizable compositions to a film due to a high melting point that is not suitable for an industrial process, or the temperature range in which liquid crystallinity is obtained may be very narrow, or solubility in a solvent generally used for an industrial process may be low. Moreover, since these low-molecular-weight polymerizable compounds and the like are synthesized by performing a plurality of steps using a synthesis method that utilizes an expensive reagent, the production cost increases.
Patent Document 25 proposes a compound (azine compound) that includes an azine linkage and has reverse wavelength dispersion in order to solve these problems.